1. Field of the Invention
The present invention relates to the preparation of hydroquinone monocarboxylates from hydroquinone dicarboxylates and, particularly, to the preparation of hydroquinone monoacetate from hydroquinone diacetate.
2. Description of the Prior Art
Hydroquinone monocarboxylates are valuable intermediates for the preparation of monohalohydroquinone (especially monochlorohydroquinone) dicarboxylates, which are themselves useful in the preparation of aromatic polyesters (cf. French Patent Application No. 79/24,135, published under No. 2,465,758 and U.S. Pat. No. 4,118,372). It has been found, in fact, that hydroquinone monocarboxylates may be selectively halogenated to monohalohydroquinone monocarboxylates, in contrast to hydroquinone which gives rise to appreciable amounts of dihalohydroquinone (cf. U.S. Pat. No. 2,743,173), which ultimately are determined as dihalohydroquinone dicarboxylates after acylation. It has also been found that hydroquinone monocarboxylates have the advantage of being capable of being halogenated in the presence of hydroquinone dicarboxylates, which themselves remain essentially inert during the halogenation. Such a property favors the halogenation of mixtures of hydroquinone monocarboxylates, hydroquinone dicarboxylates and minor amounts of hydroquinone, such as those obtained by partial acylation of hydroquinone.
No simple process is known which provides hydroquinone monocarboxylates in and of themselves. A selective route to these compounds from hydroquinone has been proposed, but its complexity deprives it of any industrial interest; in fact, this process consists of protecting one of the hydroxyl groups of hydroquinone as a functional group from which the hydroxyl moiety can readily be liberated, then acylating the other group and, finally, regenerating the first. Such a method has been applied by H. S. Olcott, J. Am. Chem. Soc., 59 392 (1937) to the preparation of hydroquinone monoacetate by reacting hydroquinone with benzyl chlorocarbonate to form the mixed benzyl and p-hydroxyphenyl carbonate which is then acetylated to mixed benzyl p-methylcarbonyloxyphenyl carbonate; in a third stage the latter is subjected to hydrogenolysis in the presence of palladium or platinum. The simplest means for providing hydroquinone monocarboxylates is the partial acylation of hydroquinone, which cannot be produced without simultaneous formation of hydroquinone dicarboxylates [cf. H. S. Olcott, Loc. cit. and D. Johnston, Chem. Ind. (London), page 1000 (1982)]. The use of such mixtures for the preparation of monohalohydroquinone monocarboxylates gives rise to an accumulation of hydroquinone dicarboxylates. The solution to the problem presented by selective production of monohalohydroquinone monocarboxylate and of corresponding dicarboxylates depends, therefore, on the development of a process for the recovery of hydroquinone dicarboxylates and the preparation of hydroquinone monocarboxylates. Cf. U.S. Pat. No. 2,588,978; published European Application, publication No. 0,060,092.